U.S. patent application number 15/458417 was filed with the patent office on 2017-09-28 for unknown.
The applicant listed for this patent is Messer Cutting Systems GmbH. Invention is credited to Murat Cetin Bayram, Thomas Muller.
Application Number | 20170274477 15/458417 |
Document ID | / |
Family ID | 58410176 |
Filed Date | 2017-09-28 |
United States Patent
Application |
20170274477 |
Kind Code |
A1 |
Bayram; Murat Cetin ; et
al. |
September 28, 2017 |
Unknown
Abstract
In known methods for through-cut detection in the thermally
assisted through cutting of a workpiece, the workpiece is subjected
to a first alternating signal. Starting therefrom, to indicate a
method which allows a fast and accurate detection of a through-cut
made, it is suggested according to the invention that the method
comprises the following method steps: a) detecting a second
alternating electrical signal caused by the first alternating
electrical signal in a measurement electrode spaced from the
workpiece, b) determining the phase shift between first and second
alternating electrical signal with output of a phase shift signal,
c) detecting a temporal evolution of the phase shift electrical
signal or a measurement variable derived therefrom in a
predetermined time interval, wherein a workpiece through-cut made
is detected in that the phase shift signal or the measurement
variable derived therefrom is in the time interval within a
predetermined fluctuation range.
Inventors: |
Bayram; Murat Cetin;
(Gross-Umstadt, DE) ; Muller; Thomas;
(Gross-Umstadt, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Messer Cutting Systems GmbH |
Gross-Umstadt |
|
DE |
|
|
Family ID: |
58410176 |
Appl. No.: |
15/458417 |
Filed: |
March 14, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B23K 2103/10 20180801;
B23K 26/048 20130101; G01R 25/005 20130101; B23K 26/0342 20151001;
B23K 2103/12 20180801; B23K 26/0622 20151001; B23K 26/082 20151001;
B23K 26/38 20130101; B23K 26/40 20130101; B23K 26/382 20151001;
B23K 2103/05 20180801; B23K 7/002 20130101 |
International
Class: |
B23K 26/38 20060101
B23K026/38; B23K 26/082 20060101 B23K026/082; B23K 7/00 20060101
B23K007/00; G01R 25/00 20060101 G01R025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 24, 2016 |
DE |
10 2016 105 560.3 |
Claims
1. A method for through-cut detection in thermally-assisted through
cutting of a workpiece, said method comprising: subjecting the
workpiece to a first alternating electrical signal, detecting a
second alternating electrical signal caused by the first
alternating electrical signal in a measurement electrode spaced
from the workpiece, determining a phase shift between the first and
second alternating electrical signal so as to produce output of a
phase shift signal, detecting a progression over time of the phase
shift signal or a measurement variable derived therefrom in a
predetermined time interval, determining that a workpiece
through-cut has been made responsive to the phase shift signal or
the measurement variable derived therefrom being within a
predetermined fluctuation range in the predetermined time
interval.
2. The method according to claim 1, wherein the measurement
electrode is kept at a constant work distance from the workpiece
during the thermally assisted through-cutting operation.
3. The method according to claim 2, wherein prior to the thermal
through-cut with the measurement electrode kept at the work
distance, an initial phase-shift signal value is determined, and
wherein the initial phase-shift signal value is in the
predetermined fluctuation range.
4. The method according to claim 3, wherein the predetermined
fluctuation range has a mean value corresponding to the initial
phase-shift signal value, and wherein the predetermined fluctuation
range is in a range of .+-.5% to .+-.15% based on the mean
value.
5. The method according to claim 1, wherein, the detecting of the
progression over time is of the measurement value derived from the
phase shift signal, and said measurement signal is derived as a
variance of the phase shift signal in the predetermined time
interval.
6. The method according to claim 1, wherein, in the detecting of
the progression over time is of the measurement value derived from
the phase shift signal, and said measurement signal is derived as a
first time derivative of the phase shift signal in the
predetermined time interval.
7. The method according to claim 1, wherein the thermally assisted
through-cutting of the workpiece is done using a treatment unit,
and an energy input is made into an incision crater, and the
through-cutting operation is monitored such that when the
progression over time detected in the predetermined time interval
exceeds a predetermined maximum fluctuation range or when in the
progression over time a peak value is detected above a
predetermined peak maximum value, the energy input into the
incision crater is changed.
8. The method according to claim 7, wherein the treatment unit
comprises a laser, and the energy input into the incision crater is
changed by changing a focal position, pulse frequency, power and/or
duty cycle of the laser.
9. The method according to claim 7, wherein the treatment unit
comprises a cutting torch, and the energy input into the incision
crater is changed by changing gas pressure of said cutting
torch.
10. An apparatus for through-cut detection in the thermally
assisted through-cutting of a workpiece, said apparatus comprising:
an alternating signal electrical signal generator generating a
first alternating signal, a measurement electrode spaced from the
workpiece and detecting a second alternating signal electrical
signal caused by the first alternating signal, a phase
discriminator for determining a phase shift between the first and
the second alternating signal, wherein said discriminator outputs a
phase shift signal, and an electronic circuit detecting a
progression over time of the phase shift signal or of a measurement
variable derived therefrom, said electronic circuit being
configured so as to identify a workpiece through-cut responsive to
the phase shift signal or the measurement variable derived
therefrom being within a predetermined fluctuation range in a
predetermined time interval.
11. The apparatus according to claim 10, wherein the measurement
electrode is supported at a constant work distance from the
workpiece during the thermally assisted through-cutting
operation.
12. The apparatus according to claim 11, wherein, prior to the
thermal through-cut with the measurement electrode kept at the work
distance, and said electronic circuit determines an initial
phase-shift signal value that is in the predetermined fluctuation
range.
13. The apparatus according to claim 12, wherein the predetermined
fluctuation range has a mean value corresponding to the initial
phase-shift signal value, and wherein the predetermined fluctuation
range is a range of .+-.5% to .+-.15% about the mean value.
14. The apparatus according to claim 10, wherein said electronic
circuit detects progression over time of the derived measurement
variable, and the derived measurement variable is derived as a
variance of the phase shift signal in the predetermined time
interval.
15. The apparatus according to claim 10, wherein said electronic
circuit detects progression over time of the derived measurement
variable, and the derived measurement variable is derived as a
first time derivative of the phase shift signal in the
predetermined time interval.
16. The apparatus according to claim 10, wherein and further
comprising a treatment unit thermally assisting the through-cutting
of the workpiece, wherein an energy input is made into an incision
crater, and the through-cutting operation is monitored such that,
when the progression over time detected in the predetermined time
interval exceeds a predetermined maximum fluctuation range, or when
in the progression over time a peak value is detected above a
predetermined peak maximum value, the energy input into the
incision crater is changed.
17. The apparatus according to claim 16, wherein the treatment unit
comprises a laser, and the energy input into the incision crater is
changed by changing a focal position, pulse frequency, power and/or
duty cycle of the laser.
18. The apparatus according to claim 16, wherein the treatment unit
comprises a cutting torch, and the energy input into the incision
crater is changed by changing gas pressure of said cutting torch.
Description
TECHNICAL FIELD
[0001] The present invention refers to a method for through-cut
detection in the thermally assisted through-cutting of a workpiece
in which the workpiece is subjected to a first alternating
electrical signal.
[0002] Furthermore, the present invention refers to an apparatus
for through-cut detection in the thermally assisted through-cutting
of a workpiece, comprising an alternating electrical signal
generator for generating a first alternating signal.
PRIOR ART
[0003] The thermal cutting of workpieces is normally carried out by
the combined use of a focused laser beam and a gas jet. Depending
on the separation mechanism, a distinction is made between
laser-beam fusion cutting, laser-beam evaporation cutting or laser
flame cutting. Laser-beam fusion cutting is widely used. Material
separation is here carried out by fusing the material by way of the
heating power of the laser beam in the area of the cutting front
and by expulsion by way of simultaneous pulse transmission of the
cutting gas jet out of the kerf. High-performance lasers,
particularly CO.sub.2, fiber, disk and diode lasers, are used,
wherein predominantly circularly polarized or unpolarized laser
radiation is used to avoid directional dependence in the absorption
behavior in contour cuts.
[0004] In the piercing or through-cutting process a first hole is
formed in the workpiece. Here, laser radiation is normally used
only in pulsed operation to pierce the material as fast as
possible, but at the same time as gently as possible. The detection
of the through-cut made poses problems. In the case of a manually
programmed piercing operation the piercing parameters, including
the typical piercing duration, are stored in a database for each
specific constellation of workpiece type and thickness. However,
due to unforeseeable material deviations and changes in the process
the piercing duration must be provided with a temporal safety
buffer. The safety buffer is to ensure that in the production
process the time between the beginning of the piercing operation
and the cutting operation is not chosen too short and that no waste
is produced. However, if the time is too long, the total time
required for processing is unnecessarily prolonged and despite the
fact that the piercing process is actually completed, further
process energy is introduced into the workpiece. This energy can
change the properties of the material and makes it difficult to
maintain a constant cutting quality in a reproducible manner.
[0005] Devices for automatically detecting the through-cut made
avoid this drawback. These devices comprise, for instance, an
optical sensor which senses the backscattered light during the
piercing operation and interprets a change in the backscatter light
above a threshold value as the completion of the piercing process.
The use of optical sensors requires, however, a certain
installation space. Moreover, the sensors are arranged either near
the workpiece, so that under separation conditions they are exposed
to high thermal stresses, or they are arranged at a distance from
the separation process, which results in an unfavorable
signal/noise ratio, so that the electrical signal of the sensor
must normally be amplified. Furthermore, optical sensors have the
drawback that there are factors in the optical path that change the
sensor signal, for instance the nozzle diameter.
[0006] Another approach is made in DE 10 2010 039 525 A1, which
discloses a method and an apparatus for through-cut detection
according to the above-mentioned type. For the automatic detection
of the through cut a capacitive distance sensor is used, as is also
employed during the cutting process for contactless distance
measurement, thereby ensuring a distance between laser nozzle and
upper side of the workpiece. In the capacitive distance measurement
system the metallic distance sensor and the metallic workpiece form
a variable capacitor which is integrated in an LC resonant circuit.
If a first alternating electrical signal is applied to the
workpiece, the capacitance of said capacitor is determined through
the distance between sensor and workpiece. When traveling over the
workpiece in the area of the aperture, the distance sensor will
detect due to the sudden change in distance that a through cut has
been formed in the workpiece.
TECHNICAL OBJECT
[0007] In this method, through-cut detection depends substantially
on the detection of the increase in amplitude in the LC generator
output signal. The amplitude level depends however on a plurality
of factors, for instance on the resistances prevailing in the
resonant circuit and the size of the interspace, but particularly
on the distance between workpiece and sensor (which is also called
"measurement electrode" in the following).
[0008] The LC generator output electrical signal often shows
background noise. This situation is aggravated by the fact that
during piercing a plasma capsule of charged particles is formed on
the upper side of the workpiece. The formation of the plasma is
detrimental to the capacitance between measurement electrode and
upper side of the workpiece, which leads to a fluctuating voltage
signal. Another reason for this are detached material parts which
are ejected from the incision crater towards the measurement
electrode.
[0009] This complicates an exact, particularly early detection of
the through cut.
[0010] It is therefore the object of the invention to indicate a
method which allows an early detection of the imminent through-cut
or the through-cut made.
[0011] Furthermore, it is the object of the invention to provide an
apparatus for performing the method.
GENERAL DESCRIPTION OF THE INVENTION
[0012] As for the method, the above-mentioned object is achieved
according to the invention by a method comprising the following
method steps:
[0013] a) detecting a second alternating electrical signal caused
by the first alternating electrical signal in a measurement
electrode spaced from the workpiece,
[0014] b) determining the phase shift between first and second
alternating electrical signal with output of a phase shift signal,
and
[0015] c) detecting a temporal evolution of the phase shift signal
or a measurement variable derived therefrom in a predetermined time
interval, wherein a workpiece through-cut made is detected in that
the phase shift signal or the measurement variable derived
therefrom is in the time interval within a predetermined
fluctuation range.
[0016] The invention is based on the idea to detect a through cut
as early as possible, preferably still in its creation process. In
contrast to known methods with an LC resonant circuit an evaluation
of the amplitude signal is omitted. Instead of this, according to
the invention a differential measurement method is employed for
through-cut detection, wherein two signal s are used and their
phase shift relative to each other is determined, namely a
measurement signal which is output by a measurement electrode and a
reference signal on which the measurement signal of the measurement
electrode is based. By comparison of the phase of measurement
signal and reference signal the phase shift signal is produced.
This is a corrected evaluation signal in which measurement errors
are eliminated, and which has a particularly good signal/noise
ratio.
[0017] For this purpose the workpiece is first acted upon with a
signal changing in time (first alternating signal). Preferably, the
first alternating electrical signal is an alternating voltage
signal U.sub.1 (t). In an electrode arranged at a distance from the
workpiece, the first alternating electrical signal produces a
second alternating signal, for instance an alternating current
signal I.sub.1,.phi. (t), which is used as a measurement signal,
and which by comparison with the first alternating signal
(reference signal) has a phase shift. To be able to detect the
phase shift as exactly as possible, the first alternating
electrical signal is used as a reference signal. The phase shift is
determined by a comparison of the first alternating electrical
signal with the second alternating signal.
[0018] It has been found that the phase shift signal depends on the
capacitance formed between measurement electrode and the workpiece.
With an increasing distance of the measurement electrode from the
workpiece the amount of the phase shift signal is increasing. At a
constant distance of measurement electrode and workpiece the
capacitance is predominantly determined by the dielectric constant
of the dielectric.
[0019] Due to the plasma capsule formed in the space between
measurement electrode and workpiece prior to the through cut, the
composition of the dielectric is permanently changing and thus that
of measurement electrode and thus the phase shift signal.
[0020] When the plasma capsule disappears, the composition of the
dielectric no longer changes and the phase shift signal is
constant. If the phase shift signal is within a predetermined time
interval within a predetermined fluctuation range, this serves as
an indicator of the through-cut made.
[0021] It has here turned out to be useful when the first
alternating electrical signal which serves as a reference signal is
first inverted for the determination of the phase shift, the
amplitudes of first and second alternating electrical signal are
matched to each other and adjusted, and the first and the second
alternating electrical signal are subsequently added. In this case,
if there is no phase shift, first and second alternating signal s
neutralize each other. However, if there is a phase shift, a phase
shift signal is obtained, the level and direction of which depend
on the phase shift. The phase shift signal changes upon a change in
distance between measurement electrode and workpiece and upon a
change of the dielectric by plasma formation in the interspace.
[0022] To avoid a distortion of the phase shift signal by distance
changes, the measurement electrode during the thermally assisted
through cut is preferably kept at a constant work distance from the
workpiece.
[0023] Advantageously, prior to the thermal through-cut with the
measurement electrode held at work distance, an initial phase-shift
signal value is determined, wherein the predetermined fluctuation
range comprises the initial phase-shift signal value.
[0024] The initial phase-shift signal value is preferably
determined before the laser beam acts on the workpiece.
[0025] It is within the predetermined fluctuation range, namely
ideally in the middle thereof. Thus, a procedure is preferred in
which the predetermined fluctuation range has a mean value
corresponding to the initial phase-shift signal value, wherein the
predetermined fluctuation range is in a range of .+-.5% to .+-.15%
based on the mean value.
[0026] It has turned out to be useful when according to method step
c) the variance of the phase shift signal in the predetermined time
interval is detected as a derived measurement variable.
[0027] However, with respect to particularly low noise and high
measuring accuracy it has turned out to be particularly
advantageous when according to method step c) the first time
derivative of the phase shift signal is detected in the
predetermined time interval and used as a derived measurement
variable.
[0028] Moreover, it is also possible with the help of the invention
to detect an imminent through-cut that has not been carried out
yet, and to react to it, if necessary. This possibility is based on
the fact that the plasma capsule in the course of the
through-cutting process and increasing material fusion in the
incision crater can disappear fully or in part. The influence of
the plasma capsule on the phase shift signal thereby changes, which
signal thereby shifts to noticeably higher or lower voltages. This
phenomenon is particularly noticeable in workpieces with rather
great material thicknesses, normally at material thicknesses above
10 mm. It depends on the workpiece thickness and the type of
material whether the shift takes place towards higher or lower
voltages.
[0029] In this respect it is intended in a particularly preferred
procedure that in the thermally assisted through-cutting of the
workpiece by way of a treatment unit an energy input takes place
into an incision crater, and that the through-cutting operation is
monitored such that when the temporal evolution detected according
to method step c) in the predetermined time interval exceeds a
predetermined maximum fluctuation range or when in the temporal
evolution according to method step c) a peak value is detected
above a predetermined peak maximum value, the energy input into the
incision crater is changed.
[0030] The energy input into the incision crater in the case of an
imminent through-cut is reduced or increased in this process. If it
is reduced, the workpiece is treated with care and a clean
through-cut is obtained. If it is increased, the through-cutting
operation is accelerated.
[0031] The energy input into the incision crater can be changed by
reducing or increasing the distance of treatment unit and
workpiece. This measure, however, also changes the phase shift
signal, so that the change of the energy input preferably takes
place via other measures.
[0032] In the event that the treatment unit comprises a laser, it
has for instance turned out to be useful when the energy input into
the incision crater is changed by changing focal position, pulse
frequency, power and/or duty cycle of the laser.
[0033] By contrast, if the treatment unit comprises a cutting
torch, the energy input into the incision crater can also be
changed by increasing or decreasing a gas pressure, preferably a
cutting gas pressure.
[0034] The introduction of the phase with reduced energy input into
the incision crater may lead to an operator message to indicate the
changed conditions to the operating personnel optically or
acoustically, so that in case of a misinterpretation a manual
intervention is possible.
[0035] As for the apparatus for through-cut detection in the case
of a thermally assisted through-cutting of a workpiece, the
above-mentioned object is achieved according to the invention by a
modification of the apparatus of the above-mentioned type, which
comprises a measurement electrode spaced from the workpiece for
detecting a second alternating electrical signal caused by the
first alternating signal, a phase discriminator for determining a
phase shift between the first and the second alternating signal,
which discriminator outputs a phase shift signal, and wherein an
electronic circuit is provided for detecting a temporal evolution
of the phase shift signal or a measurement variable derived
therefrom and is designed such that a workpiece through-cut is
identified if the phase shift signal or the measurement variable
derived therefrom is in a predetermined time interval within a
predetermined fluctuation range.
[0036] The apparatus allows an early detection of the imminent
through-cut or the through-cut made. To this end an alternating
electrical signal generator is provided that is suited to produce a
first alternating electrical signal which can be applied to the
workpiece. Preferably, the first alternating electrical signal is
an alternating voltage signal U.sub.1 (t). In an electrode arranged
at a distance from the workpiece the first alternating electrical
signal creates a second alternating electrical signal which is
detected with a measurement electrode which has a distance from the
workpiece. The second alternating signal, for instance an
alternating current signal I.sub.1, .phi. (t), and the first
alternating signal U.sub.1 (t) are present as measurement signal at
a phase discriminator which outputs a phase shift signal from which
the phase shift of both signal s can be derived. It has been found
that the phase shift depends on the capacitance formed by the
measurement electrode and the workpiece, which at a constant
distance of measurement electrode and workpiece is predominantly
determined by the dielectric constant of the dielectric.
[0037] Since prior to the through cut, plasma forms in the
interspace between measurement electrode and workpiece, the
composition of the dielectric changes and thus the capacitance
formed by measurement electrode and workpiece. Due to the changed
capacitance the phase shift signal is changed. When the through cut
has been made, the plasma capsule disappears and the phase shift
signal is constant. The electronic circuit is therefore configured
such that it monitors the temporal evolution of the phase shift
signal and evaluates it as an indicator of the through cut made if
the phase shift signal is during a predetermined time interval
within the fluctuation range, which is also predetermined.
[0038] The measurement electrode is positioned close to the cutting
process proper, which has an advantageous impact on the
signal/noise ratio. The apparatus is particularly designed for
performing the method according to the invention, so that the
relevant explanations also apply to the apparatus according to the
invention.
EMBODIMENT
[0039] The invention will now be described in more detail with
reference to embodiments and a drawing. In detail,
[0040] FIG. 1 shows a set-up of an apparatus according to the
invention for through-cut detection,
[0041] FIG. 2 shows a first diagram, in which a phase shift signal
is illustrated as a function of time,
[0042] FIG. 3 shows a second diagram, in which a phase shift signal
is illustrated as a function of time,
[0043] FIGS. 4A to 4E show, for the explanation of the method for
through-cut detection, an arrangement of workpiece and apparatus
for through-cut detection in time sequence before, during and after
a through cut,
[0044] FIG. 5 shows a laser cutting head with an apparatus
according to the invention for through-cut detection and a
shielding element,
[0045] FIG. 6 shows a schematic diagram of an apparatus according
to the invention for through-cut detection, which can also be used
for cut break-off detection,
[0046] FIG. 7 shows a cutout from the diagram of FIG. 6 in case of
a cut break-off, and
[0047] FIG. 8 shows a diagram in which a phase-shift direct-voltage
signal is illustrated during a cutting operation as a function of
time.
[0048] On the whole, reference numeral 20 is assigned to the
apparatus schematically shown in FIG. 1 for through-cut detection.
The apparatus 20 comprises an alternating signal generator 200, a
measurement electrode 207, an inverter 201, a phase discriminator
202, and an electronic circuit 250 which is designed for detecting
a through-cut made.
[0049] The apparatus 20 is part of a laser cutting machine and is
used in the thermal treatment of a workpiece 208 for monitoring a
through-cut process into the workpiece 208 and for controlling the
laser cutting machine during the through-cutting operation. The
laser cutting machine comprises a movable laser treatment unit (not
shown) with a laser cutting head 209 which has the measurement
electrode 207 fastened thereto. For the setting of a predetermined
distance of the laser cutting head 209 relative to the workpiece
surface a height sensor system (not shown) is provided that defines
the position of the laser cutting head 209 and thus of the
measurement electrode 207.
[0050] The method according to the invention will now be explained
with reference to the described laser cutting machine.
[0051] Before the beginning of a through-cutting process the
measurement electrode 207 is positioned in a first step above the
workpiece 208 such that the workpiece surface and the measurement
electrode 207 have approximately a distance from each other that
corresponds to the presumable later work distance during the
through-cutting process.
[0052] The alternating electrical signal generator 200 produces an
alternating voltage signal U.sub.1 (t) which is present at the
workpiece 208. This alternating voltage signal U.sub.1 (t) and its
electrical field have the consequence that a second alternating
signal, namely the alternating current signal I.sub.1,.phi. (t), is
produced in the measurement electrode 207. Both alternating signal
s U.sub.1 (t) and I.sub.1, .phi. (t) have the same time periods,
but differ in the phase, wherein the alternating current signal
I.sub.1, .phi. (t) is phase-shifted by the angle .phi. relative to
the first alternating voltage signal U.sub.1 (t). The magnitude of
the phase shift depends inter alia on the distance of the
measurement electrode 207 from the workpiece 208.
[0053] Subsequently, in a second step an initial value is
determined for the phase shift on the basis of the current
measurement signal s. This value will be called "initial
phase-shift signal value" in the following. For the determination
of the initial phase-shift signal value the reference signal
U.sub.1 (t) is first inverted by means of the inverter 201, i.e.
phase-rotated by 180.degree.. The inverter 201 supplies a
phase-rotated alternating current signal I.sub.1, inv (t) as the
output signal.
[0054] Both the phase-rotated alternating current signal I.sub.1,
inv (t) and the phase-shifted alternating current signal I.sub.1,
.phi. (t) are present as input signal s at the phase discriminator
202. If the alternating current signal s .sub.1, .phi. (t) and
I.sub.1, inv (t) are not phase-shifted relative to each other,
these will fully offset one another at the same amplitude level. In
the case of a phase shift, however, depending on whether I.sub.1,
.phi. (t) I.sub.1, inv (t) is leading or trailing, one obtains a
positive or negative phase shift signal which is present as an
output signal of the phase discriminator 202, here in the form of a
direct voltage signal U.sub.DC. The amount of the signal is a
measure of the phase angle .DELTA..phi. in which the phases of the
signal s I.sub.1, .phi. (t) I.sub.1, inv (t) differ. To allow a
simple comparison of the signal s I.sub.1, .phi. (t) I.sub.1, inv
(t), at least one of the signal s present at the phase
discriminator 202 is optionally pre-amplified to adapt the
amplitude level of both signal s I.sub.1, .phi. (t) I.sub.1, inv
(t) to each other.
[0055] The initial phase-shift signal value determined in this way
is stored by the electronic circuit 250 and then used, in
consideration of a tolerance value, for the definition of the
predetermined fluctuation range which is used for the subsequent
workpiece through-cut detection. The height sensor system of the
machine control is here deactivated, so that the distance between
measurement electrode 207 and workpiece 208 remains constant.
[0056] After detection of the initial phase-shift signal value, the
through-cutting process is started. An energy input in the form of
laser radiation into the workpiece area of the later through-cut is
here carried out. The laser radiation is focused in the laser
cutting head. Due to the energy input into the workpiece 208 this
workpiece is fused in the area of the later through-cut. A plasma
capsule of electrically charged particles is here formed on the
upper side of the workpiece 208. This plasma capsule causes a
change in the capacitance between measurement electrode 207 and
upper side of the workpiece 208. Moreover, the distance of
workpiece 208 from the measurement electrode 207 is changing, and
detached workpiece parts may be accelerated in the direction of the
measurement electrode 207. All of this contributes to the fact that
the phase shift of the signal I.sub.1, .phi. (t) is constantly
changing in relation to the reference signal I.sub.1, inv (t)
during the through-cutting operation. Since the capacitance between
measurement electrode 207 and upper side of the workpiece 208
changes due to the changing plasma over time, a strongly varying
phase shift signal U.sub.DC is obtained during the through-cutting
process as an output signal of the phase discriminator 207.
[0057] As soon as a through-cut through the workpiece 208 has been
carried out, the plasma capsule disappears, whereby the phase shift
signal changes suddenly into an almost constant signal value. The
substantially constant signal value of the phase shift signal after
the through-cut corresponds substantially to the initial
phase-shift signal value, so that on the basis of the phase shift
signal and its evolution a through cut can be reliably detected
both on the basis of the constancy of the value and on the basis of
its amount. To this end the temporal evolution of the phase shift
signal is detected by the electronic circuit 250 in a time interval
and evaluated. The electronic circuit 250 is designed for this
purpose such that when in a time interval of 200 ms, which is
evaluated by the electronic circuit, the phase shift signal is
within the predetermined fluctuation range of +10% around the
initial phase-shift signal value, a performed through-cut is
detected.
[0058] If this is the case, the electronic circuit 250 outputs for
instance an optical and/or acoustic indication signal, and it then
starts, based on the through cut, the cutting process, or it ends
the through-cutting process.
[0059] FIG. 2 shows by way of example a typical time curve t (in
seconds) of a phase-shift voltage signal U.sub.DC (in V) during
through-cutting through a solid material of high-grade steel with a
material thickness of 10 mm by using the apparatus of FIG. 1.
[0060] On the whole, reference signal 10 is assigned to the signal
curve. In the illustration of FIG. 2 the signal curve is subdivided
into plural sections a, b, c. Section a marks the signal section
that corresponds to the movement of the measurement electrode 207
to the initially estimated work height. When the measurement
electrode 207 has reached this work height, the apparatus will
provide a constant phase-shift signal value which is detected by
the electronic circuit 250 as initial phase-shift signal value. In
signal curve 10 this constant signal value is designated by
reference numeral 11.
[0061] Section b of the signal curve 10 represents the
through-cutting process. Particularly due to the coupling-in of
high electrical power a plasma capsule is formed on the upper side
of the workpiece, the plasma capsule being accompanied by
capacitance fluctuations and a varying phase-shift signal with high
noise.
[0062] After the through cut has been made (section c), the phase
shift signal decreases noticeably and the noise disappears, so that
a stable phase-shift signal which is approximately constant in time
is detected.
[0063] The through cut is thus noticeable due to the fact that the
phase shift signal 10 has a small fluctuation range, i.e. it is in
a time interval in a predetermined time interval within a
predetermined fluctuation range, and that it declines to the
initial phase-shift signal value 11. Taken together, both criteria
allow a comparatively fast and reliable statement on the through
cut.
[0064] For the specification of the fluctuation range the
previously determined initial phase-shift signal value is used.
Said value is illustrated in FIG. 2 by the broken line 12 and gives
a good approximation for the target value to be expected. In the
embodiment the predetermined fluctuation range is .+-.10% around
the initial phase-shift signal value.
[0065] FIG. 3 shows another time curve 30 t (in seconds) of the
phase-shift voltage signal U.sub.DC (in V), as has been measured
during the through-cutting of a solid material of high-grade steel
with a material thickness of 15 mm.
[0066] The measurement electrode (207) and the laser cutting head
were here initially held at a fixed position relative to the
workpiece surface, namely at a work distance of about 3.5 mm. The
time curve 30 of the phase-shift voltage signal U.sub.DC reveals
three sections x, y, z, which will be described in detail
hereinafter.
[0067] Section x (corresponding to the phase section a in the
diagram of FIG. 2) shows the phase shift signal as obtained in the
movement of the measurement electrode and the laser cutting head to
the work height. The signal curve in section x correlates with the
distance changes of the measurement electrode relative to the
workpiece surface.
[0068] As soon as the work height and the constant signal value 11
have been reached, the energy input into a through-cut area of the
workpiece is started. A pulsed laser is used for this with 4 kW
power at 25% duty cycle and a pulse frequency of 70 Hz.
[0069] Due to the energy input a plasma capsule is formed on the
workpiece surface. As has already been described with respect to
FIG. 2, this leads to a varying phase-shift signal with high
background noise.
[0070] However, a look at section y in its entirety reveals that
the phase shift signal shows a drift towards lower voltages in the
course of time. Specifically, two sub-areas y.sub.1, y.sub.2 can be
seen that differ in their phase-shift mean value. In sub-area y1
plasma formation substantially takes place on the workpiece
surface. With time, however, an increasing material fusion is
observed in the through-cut area, accompanied by a spatial shift of
the plasma formation and the plasma capsule towards the middle of
the workpiece. This changes the influence of the plasma capsule on
the phase shift signal, so that the signal shifts towards lower
voltages (area y.sub.2). This phenomenon is particularly strongly
noticed in workpieces with rather great material thicknesses,
normally at material thicknesses above 10 mm.
[0071] The change of the phase shift signal in area y.sub.2 can
advantageously be used for the control of the laser. Due to the
fact that in the course of the through-cutting process the plasma
capsule spatially shifts towards the middle of the workpiece, it is
possible upon occurrence of rather great phase-shift signal
fluctuations and/or a phase shift signal drift to adapt the
performance parameters of the laser or preferably to change the
control of the laser such that the focal position of the laser
follows the changed plasma position. This can for instance be
done
[0072] a) by reducing the work height, or
[0073] b) preferably, by changing the focal position, for instance
by shifting the focal position towards the bottom side of the
workpiece.
[0074] Alternatively, it is also possible to change the power of
the laser, its pulse frequency, duty cycle or the gas pressure.
[0075] The above-mentioned measures help to achieve a particularly
high through-cut quality.
[0076] For the explanation of the method for through-cut detection,
FIGS. 4A-4E show an arrangement of workpiece and apparatus for
through-cut detection in time sequence before, during and after a
through-cutting operation.
[0077] FIG. 4A shows an arrangement of workpiece 208 and a laser
cutting head 209 on which a measurement electrode 207 is mounted.
The arrangement in FIG. 4A detects the initial phase-shift signal
value. To this end the laser cutting head 209 is located at work
height; an energy input into the workpiece does not take place.
[0078] An alternating voltage signal U.sub.1 (t) which in the
measurement electrode 207 produces a phase-shifted second
alternating signal, namely the alternating current signal (t), is
applied at workpiece 208. The initial phase-shift signal value is
determined based on both alternating signal s U.sub.1 (t) and
I.sub.1, .phi. (t).
[0079] FIG. 4B shows the same arrangement during an energy input by
way of laser radiation 400 into the workpiece 208. A plasma capsule
210 has formed on the upper side 404 of the workpiece 208. The
plasma capsule 210 has an influence on the phase shift of the
alternating signal s U.sub.1 (t) and I.sub.1, .phi. (t) and thus on
the phase shift signal. The phase shift signal as a measure of
distance A shows some noise during this method step, as is e.g.
shown in FIG. 2, section b, or in FIG. 3, section y.
[0080] As shown in FIG. 4C, an incision 403 into the workpiece 208
is increasingly formed during the through-cutting process. The
cutting point 401 shifts away from the measurement electrode 207
towards the bottom side 402 of the workpiece. At the same time the
height with which the plasma capsule 210 projects over the upper
side 404 of the workpiece 208 is decreasing; the apparent distance
A becomes greater again. This process is accompanied by a drift of
the phase shift signal. Such a drift can for instance be seen in
the phase shift signal 30 of FIG. 3, section y.sub.2. If a drift is
detected in the phase shift signal, this can be used as a basis for
a new focusing of the laser.
[0081] FIG. 4D shows the arrangement of FIG. 4C in which the focal
position of the laser beam 400 is shifted towards the bottom side
402 of the workpiece. Such an adaptation of the focal position has
the advantage that a particularly high cutting quality is
obtained.
[0082] FIG. 4E shows a through cut. The laser beam 400 passes
through the workpiece in the area of the through cut
unimpededly.
[0083] FIG. 5 shows the laser cutting head 209 in more detail. The
laser cutting head 209 is subdivided into an upper part 501 and a
lower part 502. The upper part 501 is connected to the mass of a
laser cutting machine (not shown). The lower part 502 comprises a
cutting nozzle 504 and the measurement electrode 207 mounted
thereon. If an alternating voltage is present at a workpiece 208,
said voltage produces a phase-shifted alternating signal in the
measurement electrode 207. Since the upper part of the laser
cutting head 209 is connected to the mass of the laser cutting
machine, it is necessary that the upper part 501 and the lower part
502 of the laser cutting head 209 should be electrically separated
from each other by an insulator 503.
[0084] The accuracy of the detected phase-shift signal may be
impaired by irregularities of the workpiece 208 or by parasitic
capacitances. Irregularities in this sense may for instance be
caused by the workpiece edge, the workpiece shape or cutouts from
the workpiece. Parasitic capacitances may be due for instance to
the chamfer position of the laser cutting head 209 during the
through-cutting process and the associated approach of the laser
cutting head 209 to the workpiece 208. These effects may impair the
accurate detection of a workpiece through-cut.
[0085] To minimize such influences on the measurement result, here
the phase shift signal, the laser cutting head 209 is provided with
a shielding element 506. The shielding element 506 is designed as a
shield electrode and assigned to the cutting nozzle 504. It is
conical and surrounds the cutting nozzle 504 along its lateral
surfaces. This has the advantage that the laser cutting head 209
can also be arranged above inner contours that have already been
cut, or near the workpiece edge, without the measurement result
being impaired considerably. Moreover, the position of the laser
cutting head 209 relative to the workpiece 208 can be changed
during the through-cutting operation without substantially
impairing the detection of the through cut.
[0086] The above-described apparatus according to the invention can
simultaneously be used for detecting a cut break-off. For instance,
a cutting operation directly follows the through-cutting operation.
In this case the apparatus can be used as a cut break-off detection
apparatus.
[0087] FIG. 6 shows a schematic diagram of an apparatus 20
according to the invention of FIG. 1, which can be used as a cut
break-off detection apparatus.
[0088] The apparatus 20 comprises an alternating signal generator
200, a measurement electrode 207, an inverter 201, a phase
discriminator 202, a control unit 203 and three independent
electronic circuits 204, 205, 206.
[0089] The apparatus 20 is part of a laser cutting machine (not
shown), as is e.g. used for cutting a flat workpiece 208 of metal,
preferably of high-grade steel, aluminum, copper or brass.
[0090] The cut break-off detection method will be explained
hereinafter with reference to the above-described laser cutting
machine.
[0091] First of all the workpiece 208 is subjected to an
alternating voltage signal U.sub.1 (t). To this end the alternating
signal generator 200 produces the alternating voltage signal
U.sub.1 (t) which is applied to the workpiece 208 and is
subsequently used as a reference signal .
[0092] The alternating voltage signal U.sub.1 (t) produces an
alternating current signal I.sub.1, .phi. (t) in measurement
electrode 207. Both alternating signal s U.sub.1 (t) and I.sub.1,
.phi. (t) have the same time periods, but differ in the phase, with
the alternating current signal I.sub.1, .phi. (t) being
phase-shifted by the angle .phi. relative to the first alternating
voltage signal U.sub.1 (t). The magnitude of the phase shift
depends inter alia on the distance of the measurement electrode 207
from the workpiece 208. The alternating current signal I.sub.1,
.phi. (t) is detected by means of the measurement electrode
207.
[0093] Under normal cutting conditions the distance between
measurement electrode 207 and workpiece 208 is kept as constant as
possible, apart from control deviations, by the height sensor
system. Although the alternating current signal I.sub.1, .phi. (t)
resulting therefrom shows some noise, it has a phase shift almost
constant in time relative to the reference signal U.sub.1 (t).
[0094] For the determination of the phase shift, the reference
signal U.sub.1 (t) is first of all inverted by means of the
inverter 207, i.e. phase-rotated by 180.degree.. The inverter 201
supplies a phase-rotated alternating current signal I.sub.1, inv
(t) as the output signal.
[0095] Both the phase-rotated alternating current signal I.sub.1,
inv (t) and the phase-shifted alternating current signal I.sub.1,
.phi. (t) are present at the phase discriminator 202 as input
signal s. The phase discriminator 202 also includes a rectifier. If
the alternating current signal s I.sub.1, .phi. (t) and I.sub.1,
inv (t) are not phase-shifted with respect to each other, these
will fully neutralize each other at the same amplitude level. In
the case of a phase shift, however, depending on whether I.sub.1,
.phi. (t) I.sub.1, inv (t) is leading or trailing, one obtains a
positive or negative phase shift signal in the form of the
direct-voltage signal U.sub.DC. The amount of the signal is a
measure of the phase angle .DELTA..phi. in which the phases of the
signal s differ. To allow a simple comparison of the signal s, at
least one of the signal s at the phase discriminator 202 is
optionally pre-amplified (not shown) to adapt the amplitude levels
of the two signal s to each other.
[0096] Subsequently, the phase shift signal U.sub.DC is compared by
the control unit 203 with a predetermined upper and lower limit
value.
[0097] During the normal cutting operation the limit values are
normally not exceeded or fallen short of. However, in the event of
a cut break-off, a plasma capsule 210 is formed on the upper side
of the workpiece 208. This plasma capsule 201 is mainly created by
the coupling-in of high power peaks into the workpiece 208.
[0098] FIG. 7 shows the laser cutting head 209, the workpiece 208
and the plasma capsule 210 in the event of a cut break-off. The
evolving plasma capsule 210 causes a change in the capacitance
between measurement electrode 207 and upper side of the workpiece
208. Moreover, detached workpiece parts are accelerated due to the
kerf, which no longer penetrates the material, towards the nozzle
and the measurement electrode 207, respectively. This results in a
changed phase shift of signal s I.sub.1, .phi. (t) and I.sub.1, inv
(t). Since the capacitance between measurement electrode 207 and
upper side of the workpiece 208 changes and varies due to the
variable plasma over time, a varying phase-shift signal U.sub.DC
which is used for the detection of the cut break-off is also
obtained as the output signal of the phase discriminator 202. To
this end the phase shift signal is monitored by the control unit
203 as to whether it exceeds or falls below an upper or lower
limit. In the event that a respective limit value is exceeded or
not reached,
[0099] the separation speed is reduced by means of the electronic
circuit 204,
[0100] the measurement electrode is set by means of the electronic
circuit 205 to a predetermined fixed position, and
[0101] an optical and acoustic alarm signal is sent by means of the
electronic circuit 206.
[0102] FIG. 8 shows by way of example a temporal evolution of the
phase-shift voltage signal U.sub.pc for a good cut (section I), an
imminent cut break-off (section II) and after the cut break-off has
taken place (section III). The phase shift signal is marked with
reference numeral 80.
[0103] Prior to the cut break-off, phase shift signal 80 exhibits
noise that is standard during the cutting operation. Nevertheless,
the phase shift signal 1 in section I is substantially constant and
varies around a mean value with only a small deviation. An imminent
cut break-off leads to an amplification of the phase shift signal
80 in section II up to maximum deflection in section III.
[0104] To successfully counteract an imminent cut break-off and
thereby to avoid a cut break-off, it is important to detect a
beginning cut break-off as early as possible. The use of the phase
shift signal allows early cut break-off detection particularly in
section II. The upper limit value U.sub.lim, 1 and the lower limit
value U.sub.lim, 2 are chosen such that they allow early
detection.
* * * * *